This invention relates to the blast furnace smelting of zinc, and more particularly to the introduction of a blast of air into such a furnace.
The smelting of zinc in a blast furnace is now a well known industrial technique, having been practised on a production scale since the early 1950's (see, for example, "Application of the Blast Furnace to Zinc Smelting" by Morgan and Woods, published in Metallurgical Reviews 1971, 16 (156), November, pages 161-174). In the process air, or oxygen-enriched air, is introduced through tuyeres near the furnace bottom and a zinc vapour-bearing gas is taken off through an offtake near the top of the shaft and zinc is condensed from this gas by means of a condenser which is irrigated with a spray of molten lead droplets.
One problem associated with the blast furnace smelting of zinc is the loss of zinc (as zinc oxide) in the slag falling to the furnace bottom. This slag may contain as much as 10% zinc as zinc oxide and this clearly represents an overall reduction in the recovery of zinc from the oxidic material fed to the furnace. Various attempts have been made to reduce the "zinc-in-slag" loss but the solubility of zinc oxide in the other slag-forming constituents and a requirement for the best possible fuel economy have made this difficult. It has been proposed to recover zinc from lead blast furnace slag by a "slag-fuming" process in which air and reducing material are blown into molten slag and the zinc fumed-off and oxidised, but this is not very acceptable for zinc blast furnace slags since it would require capital expenditure on additional plant and would be expensive in terms of fuel consumption.
British Pat. No. 1,458,869 describes one way of reducing zinc loss in slag, by blasting air into the slag pool through tuyeres downwardly-inclined at an angle of between 12.degree. and 20.degree. to the horizontal. This does provide some reduction in the zinc content of the slag but still better gas/slag contact is desirable. It is believed that in order to maximize the reduction of the zinc in the slag at the furnace bottom it is necessary to obtain the best possible contact between carbon monoxide-containing gas and a highly-dispersed molten slag phase. Thus two things are desirable:
1. Maximum dispersion of molten slag by the air blast, and
2. Slag droplets driven up into the furnace charge from where they fall, by gravity, counter-current to a rising gas stream containing carbon monoxide.
The carbon monoxide is generated by reaction of oxygen in the air blast with carbon, either in slag or in coke lumps, at the furnace bottom, or by the reaction of carbon dioxide with carbon.
We have now discovered that the physical contact between slag and gas can be increased by the employment of a specially-designed tuyere according to the present invention.